Amiloride kills malignant glioma cells independent of its inhibition of the sodium-hydrogen exchanger

Previously, we demonstrated that malignant glioma cell lines have increased intracellular pH (pH_i) as a result of increased activities of the type I sodium/hydrogen exchanger (NHE1). This alkalotic pH₁ of 7.2 to 7.4 is favorable for augmented glycolysis, DNA synthesis, and cell cycle progression. Conversely, reductions in pH_i have been associated with reduced rates of proliferation in transformed cell types. The effects of reducing pH_i directly and by NHE1 inhibition on human malignant glioma cells were systematically compared with those on primary rat astrocytes. Neither cariporide, nor direct acidification to pH_i 6.9 altered the proliferative rates or viabilities of human U87 or U118 malignant glioma cell lines. However, amiloride significantly impaired glioma cell proliferation and viability while not affecting astrocytes at concentrations (500 μM) that exceeded its inhibition of NHE1 in glioma cells (IC₅₀ = 17 μM). Preventing a reduction of pH_i did not alter the drug's antiproliferative and cytotoxic effects on glioma cells. These findings indicated that amiloride's cytotoxic effects on glioma cells are independent of its ability to inhibit NHE1 or to reduce intracellular pH. The amiloride derivative 2,4 dichlorobenzamil (DCB) inhibits the sodium-calcium exchanger (NCX) and was both antiproliferative and cytotoxic to glioma cells at low doses (20 μM). By contrast, KB-R7943 [(2-[2-[4-nitrobenzyloxy]phenyl]ethyl)- isothioureamethanesulfonate] preferentially blocks sodium-dependent calcium influx by NCX (reverse mode) and was nontoxic to glioma cells. It is proposed that DCB (20 μM) and amiloride (500 μM) impair calcium efflux by NCX, leading to elevations of intracellular calcium that initiate a morphologically necrotic, predominantly caspase-independent glioma cell death.